Salmonella enterica subspecies Typhimurium is a natural pathogen of mice that establishes persistent, systemic infection. Salmonella generally reside within professional phagocytes, typically macrophages, which is critical for the development of chronic infection. However, it is unclear how Salmonella can survive within a cell-type that evolved to destroy pathogens. Our lab has recently demonstrated that during persistent murine infections, S. Typhimurium (STm) can reside within macrophages that are hemophagocytic. Hemophagocytic macrophages (HM?s) are characterized by the ingestion of viable cells of the hematopoietic lineages, and are clinically associated with human Typhoid fever. Cell culture assays have demonstrated that S. Typhimurium preferentially survive within HM?s and that replication within HM?s is dependent upon two type-three secretion systems. One way HM?s differ from resting or activated macrophages is that these cells accumulate vacuoles that appear to contain fatty acids. Previous studies have suggested the importance of S. Typhimurium acquisition of host cholesterol, which accumulates in Salmonella containing vesicles of cell culture macrophages. At least three T3SS secreted proteins are implicated in lipid regulation in host cells and are important for S. Typhimurium intracellular survival in macrophage-like tissue culture cells. I hypothesize that specific T3SS effector proteins mediate bacterial survival in HM?s by promoting cholesterol accumulation. Therefore, the Specific Aims of this proposal are (1) to establish whether SseJ mediates STm utilization of esterified cholesterol; (2) to establish the amount and nature of lipids in STm-infected HM?s; and finally, (3) to determine whether decreasing host serum cholesterol attenuates STm pathogenesis in vivo. The overall aim of this grant proposal is to further elucidate the complex host and Salmonella interactions that regulate pathogenesis of disease. Definition of the mechanisms by which S. Typhimurium utilizes host cholesterol for survival in HM?s will allow for understanding how persistent infections can develop in a healthy host, and these results may ultimately suggest novel targets for prevention and/or treatment strategies of Salmonella- induced disease. )